With the increasing current consumption in motor vehicles, such type of measuring arrangements (current sensors) and measuring methods are increasingly indispensable since the state parameters of the battery must be determined at every point in time by continuously measuring the battery current. Parameters such as, for example, “State of Health,” “State of Charge” or “State of Function” are intended to enable a differentiated statement about the functional capability of the battery in all operating conditions possible. These state parameters thus form the basis for an intelligent battery-energy management in the vehicle.
In individual vehicles, such measuring systems are already in use since 2001 and are described, for example in the article by Achim Scharf, “En Route to the Car of the Future,” “Power Electronics Europe Magazine,” issue Apr. 4, 2001. These systems are still very expensive and consist of a large control unit in which all the calculations are carried out on the basis of a stored battery model. Furthermore, the control unit also controls a large part of the actions derived from these calculations in the vehicle electrical system. A current sensor is integrated into this control device. The battery cable is guided though the control unit in such a way that the current can be measured in the control unit.
The costs of the solution described above are by far too high for a widespread use of battery management systems. Therefore, increasingly more manufacturers are changing their designs to use control units existing inside the vehicle electrical system for the calculations from the battery model and the control functions derived therefrom. Presently, in every vehicle there already exist several control units, which comprise a microprocessor of high computing power. Therefore in order to integrate said control functions into such a control unit, the existing microprocessor must at best be replaced by a one that is slightly more powerful. The corresponding additional costs are considerably lower than those for an additional control unit. The corresponding designs then require an extremely cost-effective current measuring module which is attached to the battery, to the pole shoe of the battery or to the battery cable and at all times provides the vehicle electrical system with information about the actually flowing battery current using a standardized interface (e.g. CAN bus).
Extremely high demands are made on this current measurement due to the fact that on the one hand currents in the range of 1000 A can temporarily flow during start-up at low temperatures. On the other hand in case of a parked vehicle, current flowing in the range of few 10 mA contributes considerably to the loss of charge of the battery over long periods of time and hence must be measured with sufficient precision. No classic current sensor is in a position to detect this dynamic range of a few mA to 1000 A with sufficient precision.
Current sensors, which are in a position to detect currents over a large dynamic range with a precision of 1% or higher are closed, loop sensors. A typical embodiment of such a sensor is illustrated in FIG. 1. It contains a magnetic core provided with an air gap. A magnetic field probe 2 is arranged in the air gap of the magnetic core. Said probe converts the magnetic field in the air gap into an electric signal which controls the current through a compensation coil 3 that is wound on the core preferably in the region of the magnetic field probe. A primary conductor 4 whose current is to be measured is guided through the magnetic core. The control electronics of the current sensor ensures that the magnetic flux supplied to the magnetic core using the current of the primary conductor is compensated at all times by the field of the compensation coil. The target value of the control circuit to be registered by the magnetic field probe is thus of zero field strength. The magnetic field probe operates as a zero field detector. In the compensated state, the current through the compensation coil Ikomp is directly proportional to the current to be measured IPrim. If the compensation coil N has windings, then the equation IPrim=N Ikomp holds true.
The magnetic field probe 2 ought to have a very small, preferably vanishing offset. Offset refers to the output signal of the probe when zero magnetic fields are present. If the probe signal in this case also is not zero, the probe has an offset that causes a distinct error of the sensor in the range of small currents.
A Hall IC can be used as a magnetic field probe. Hall ICs normally have a very distinct offset that can lead to measurement errors of the current sensor in the range of 0.5 A. However, it is possible to largely eliminate this offset using electronic measures on the Hall IC or by calibrating the current sensor.
The use of a magnetic probe, for example according to the patent application EP 0 294 590 is easier and more precise, especially when it comes to the temperature-dependence of the current sensor. Since the evaluation electronics scans the symmetry of a soft-magnetic metal strip, this magnetic field probe operates as a zero field detector in a practically offset-free and temperature-independent manner. Such a sensor is indicated schematically in FIG. 1 as a magnetic field sensor 2.
Furthermore, a magnetic core 1, which is used according to the patent application EP 0 294 590 in combination with a magnetic probe, is preferably composed according to the patent application EP 1 010 014 of two or more parts, which are joined together in a partly overlapping manner and which have a pocket in the region of the air gap for the magnetic field probe 2. The current sensor can thus be mounted by assembling the core over randomly shaped conductors and the probe is protected from external interfering magnetic field influences.
The hysteresis characteristics of the magnetic core 1 determine the precision of such a current sensor in the range of small currents, provided that the magnetic field probe 2 has no offset. The magnetization curves of practically every soft-magnetic material, especially the magnetization curve of NiFe materials that are mostly used here for cost-related reasons, have a distinct hysteresis. That is, in such a core even without external magnetic field, a remnant magnetic flux is preserved whose strength depends on the previous history of the magnetic core. Previous history means the field strengths and field directions to which the core was exposed before the measurement in case of low currents.
In case of a current sensor illustrated in FIG. 1, thus even at zero primary current, this remanance causes a certain magnetic field on the probe and thus simulates a primary current for the current sensor where said primary current is not actually present. In case of low currents, this measurement error lies typically in the range of 50 and 100 mA if the current sensor has previously detected currents close to the upper measurement range limit or even beyond that. Measured error curves during the modulation of the current sensor to beyond the upper measurement range limit are illustrated in FIG. 2. This modulation drives the sensor core partly until saturation occurs and thus causes the strongest remanance when the primary current retracts to zero.
The measured curves illustrated in FIG. 2 are recorded using current sensors provided with an offset-free magnetic probe according to the patent application EP 0 294 590. However, they would occur in the similar fashion in case of current sensors provided with Hall IC as the magnetic field probe, if the offset of the Hall IC, which is generally considerably greater than the offset caused by the remanance of the magnetic core, is eliminated.
The remanance of the magnetic core thus in principle limits the measurement range of a closed loop sensor downwards.
Therefore, the current sensor used in the described battery management system of the Power Electronics Europe Magazine, issue Apr. 4, 2001 is provided with a two-level design. The battery current sensor illustrated schematically in FIG. 3 comprises an additional low current level 5 in addition to the closed loop sensor 1 to 3 that operates here as a high current level. This low current level is designed as a wound ring core made of amorphous soft-magnetic metal on the basis of the principle described in the patent application EP 0 960 342. It operates with alternating magnetic reversal, is thus offset-free and has a very high resolution in the range of small currents. Such sensors are however not suitable for measuring high currents, thus making the two-level design absolutely necessary. The disadvantage of this existing battery current sensor that technically meets all requirements is therefore this two-level design having two varying functional principles and that doubles both the expenditure of the magnetic module and also of the evaluation electronics.
Other solutions for a battery management sensor are exclusively based on a shunt resistance that is inserted into the battery current path together with the corresponding measurement and amplification of the voltage drop on this shunt resistance. However, this solution that initially appears to be impressive and simple, also has serious disadvantages: high currents cause a strong heating of the resistance that is additionally inserted into the circuit.
On the other hand, in case of low currents, the voltage drop to be measured is so low that the measurement can be easily disturbed by electromagnetic interferences, and that sufficient precision can be ensured only by using very expensive electronic circuits. This problem is further intensified when measuring the current on the plus side of the battery since the possible fluctuation of the reference potential with the battery voltage is greater than the voltage to be measured by several powers of ten. Therefore, these solutions require either very high electronic expenditure or they do not meet the requirements related to precision and interference resistance.
Another solution of the problem is feasible, which according to the schematic illustration in FIG. 4 consists of the combination of a shunt resistance and the low current sensor illustrated in FIG. 3. By using the magnetic measurement in case of small currents, it is possible to design the resistance value of the shunt to be lower. In this manner, the heating problems in case of high currents can also be reduced. In spite of that the voltage drop on the lower end of the shunt measurement range can still be kept higher by one to two orders of magnitude than in case of the exclusive use of a shunt. Therefore, the measurement is considerably fail-safe and requires low electronic expenditure.
Although this solution appears to be considerably cheaper than the previously described solution, it requires in addition to the magnetic measuring system, a special shunt that must be suitable for currents up to 1000 A or higher. Such shunts are integrated into a current bar using expensive joining methods and are therefore not particularly cost-effective. The current bar must be inserted into the circuit using screwed connections or welded connections. The use of an existing conductor piece by which additional connections in the circuit would have been avoided, is not possible.